Rotary encoders provide variety

Frank J. Bartos, CONTROL ENGINEERING -- Control Engineering, 7/1/2000

The most common feedback device used in motion control systems is the rotary optical encoder. Comments in this 'extra section' pertain to that encoder type. Typical encoder applications encompass machine tools, web handling equipment, robotic and vision systems, packaging machines, conveyors, gantry cranes, storage and retrieval systems, etc.

As mentioned in the main article, a standard way of increasing an incremental encoder's resolution is to count the leading and trailing edges of the quadrature signals generated. This function is actually done external to the encoder by a counter or controller. Another method to increase resolution is interpolation or internal electronic subdivision of the encoder's base resolution. Typical multiplication factors obtained by interpolation are in the 2-20X range. Quadrature and interpolative methods can also be combined to further multiply the basic encoder resolution, but practical limits must be observed. An extremely high output of pulses could overwhelm the bandwidth of the control system.

Electronic interfaces are important if the encoder is to work properly with external controls. Two common types of output interfaces-open-collector and differential line driver-are discussed in the main article. In addition, the robustness of the output signals must be assured through electrical isolation. Auxiliary products exist for this purpose. For an example of a new optical isolator module specifically for incremental encoders, see the July 2000 issue's product section.

Encoder varieties, features, options

Rotary encoders are made in a variety of styles. Size range runs the gamut from miniature units that cater to tight space requirements crucial in many applications to large, heavy-duty models that resemble sizable electric motors. It's even possible to transmit power through some heavy-duty encoders that have a dual-shaft extension feature. Kit-type encoders come with all the parts needed, except a housing; the motor or machine structure into which the parts are assembled provides the housing.

In an unusual configuration, a ring-shaped encoder wraps a precision linear scale into a circular form for convenient compact mounting. The unit is combined with a miniature read head. (See product examples below.)

Encoders must contend with the harsh conditions of the applications they serve. These include high temperature, moisture, and vibration, along with EMI and related adverse electromagnetic effects. Temperature tolerance of some encoders is surprisingly high. Models are available for continuous operation up to 105 °C (221 °F). Capability to work up to 125 °C (257 °F) is said to be under development. However, more typical for industrial usage is the 70-85 °C range. At the low-temperature end, 0 to -10 °C is typical capability.

Proper sealing is likewise important. NEMA 4 (IP65) is typical for industrial usage, however, NEMA 4X, 6, or even 13 is available. Encoders can also be designed to be intrinsically safe and explosion proof.

Rotational speed capability of the encoder must match that of the motor being monitored. In most cases this requirement is met, with encoders available up to 30,000 rpm operation. Of course, such high speeds carry stringent bearing and mounting design requirements. Precision rolling-element bearings must be used.

In recent years, the hollow-shaft housing version has been added by virtually all encoder manufacturers. This design opens the encoder's centerline to direct access by electric, hydraulic, pneumatic, or other process lines, simplifying the motion system installation and making it more compact. Depending on the encoder model, the hollow-shaft opening can be quite large. This allows convenient direct mounting to larger diameter motor shafts.

Other mounting methods include shaft-to-shaft connection to the motor (or rotary load) via a coupling, mounting to the stator, bracket mounting, and tether mounting. Standard electrical connections from the encoder to the motor include cable, pin-style connector, terminal block, etc.

A brief sampling of products follows to illustrate the rich variety of rotary encoders available.

Rotary ring encoder Schaumburg, Ill.-RGR Rotary Ring Encoders wrap linear optical encoder technology into a circular form for 360° rotary capability. A 20-µm pitch scale forms the periphery of the low-profile (10 x 10 mm cross section) stainless-steel ring that comes ready to mount in tight spaces. RGR encoders are available in 3-, 4-, 6-, and 8-in. (75, 100, 150, and 200 mm) ring diameters, have matching read heads, and come with digital or analog incremental output. The rings come with a precounted number of lines; repeatable reference marks are optional. RGR ring encoders operate at speeds of more than 2,000 rpm, with resolutions said to be down to 0.2 arc sec. Renishaw Inc.

`Smallest' high-resolution encoder Troy, N.Y.- R119 encoder, measuring just 0.75 diameter x 0.9-in. long, is available in a conventional shaft-mount and blind, hollow-shaft version with flexible tether mount. The 'thumb-sized' optical encoder generates quadrature square waves at up to 10,240 cycles per revolution (40,960 counts/rev after the user's 4X quadrature decode). A ¼-cycle gated, once-per-revolution index signal is standard. Output device is an RS-422 differential line driver. Gurley Precision Instruments

Modular encoders Schaumburg, Ill.-ERO 1200 and 1400 Series modular rotary encoders consist of a disk/hub assembly-mountable to the measured shaft-and a scanning unit that is attached to it. These encoders feature several popular electronic interfaces used for TTL signals. A 1-V pp interface, which outputs signals that allow higher interpolation, is also available for fine measurement steps needed in speed control. ERO Series comes with 4-12 mm shaft diameters and various line counts and speed capabilities. Heidenhain Corp.